Andy Duncan yesterday ended the growing uncertainty over his position as chief executive of Channel 4 by announcing he would quit the group before the end of the year.

Mr Duncan said the decision about his departure was "mutual". He added: "Clarifying my intentions will allow everyone at Channel 4 to get on with the job at hand without the distracting speculation of recent weeks".

Claire Enders, of Enders Analysis, said Mr Duncan was a "political victim". She said: "The efforts to fix C4's future were so demanding. He was made a fall guy for mission impossible." It is also understood that personal tensions helped speed his departure.

In his remaining months, Mr Duncan said he wanted to secure an "update of our remit and the completion of a number of exciting commercial partnerships currently under discussion."

The broadcaster's chairman, Luke Johnson, praised Mr Duncan's "effective leadership", adding that the broadcaster "has enjoyed record creative and commercial success in the last five years while skillfully navigating the seismic upheavals in digital media".

C4 doesn't expect a successor to be named until Mr Johnson, who steps down in January, is replaced.

During his tenure, Mr Duncan helped C4 boost its share of total viewing from 10 per cent to a historic high of 12 per cent. Its share of UK advertising has grown from 20 per cent to 24 per cent. He joined the broadcaster from the BBC in July 2004.

Mr Duncan said it was "a great five years, during which time we've punched well above our weight, out-performing our immediate rivals both creatively and commercially". Ms Enders said: "C4 is in the best shape of all its rival commercial broadcasters. It has the strongest balance sheet, a good creative record and didn't diversify into unwise sectors. He was a good steward but the broadcaster is a political football."

Part of the growth has been driven by its push into digital.

Insiders said Mr Duncan's fate was sealed in the wake of the publication of Digital Britain, the government's blueprint to secure a public service broadcasting rival to the BBC and bring Britain into the digital age.

C4 has been searching for a way to fill a funding gap expected to hit £150m a year by 2012. Mr Duncan's preferred option was a joint venture with the BBC's commercial arm, BBC Worldwide, which Lord Carter's report supported. But the negotiations have stalled.

Mr Duncan had a strained relationship with Mr Johnson, and after Digital Britain was deemed not to have provided the "silver bullet", other board members also turned on him.

Mr Duncan said: "The publication of the Digital Britain report was also a natural moment for me to take stock. Channel 4 is facing a further period of change, with a fresh regulatory cycle looming and with the cancellation of Big Brother signalling the most significant creative renewal in our history."

In the frame: Channel 4 contenders

In the wake of Andy Duncan's departure, the industry was already laying odds on the runners and riders to replace him. According to one media expert, that field is set to widen after ITV unveils its new chief executive. Former BSkyB chief executive Tony Ball is expected to be named as Michael Grade's replacement as early as next week.

The clear favourites are those who are expected to be passed over for the job at ITV. This includes Peter Fincham, the director of television; Rupert Howell, the commercial director; and John Cresswell, the broadcaster's chief operating officer.

One industry insider said: "They will probably also have a look at Dawn Airey, and see if she is happy with life as chief executive and chairman of Five." Longer shots yesterday included Lord Stephen Carter, who backed C4's joint venture with BBC Worldwide in his Digital Britain report and announced he would leave the government after its publication. Other candidates mooted included the BBC Worldwide's chief executive, John Smith, and Rob Woodward, the chief executive of STV, who has a "good reputation but not a lot to play with," one source said.

One industry expert said: "I hope the broadcaster goes for someone from within the television sector, rather than a businessman or a politician. It's what Channel 4 needs."

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